Contribution of Atmospheric Circulation Changes and Anthropogenic Warming to the Increase of Warm Season Fire Weather Risk in California

Abstract:

Recent increasing wildfire activity in the Western United States raised widespread public concerns. Previous research efforts have concluded that anthropogenic warming plays a major role in increasing fire risk using attribution analysis based on model simulation and projection. This study shows how fire risk has changed in California, and also explore the cause of increased fire weather risk by estimating the contribution of circulation changes and only using observational data, including fire perimeters data, gridMET, and multiple reanalysis datasets (e.g., ERA5, MERRA2, CFSR). Our results demonstrate that there was a significantly positive trend of vapor pressure deficit (VPD), an important fire weather risk index, in all days as well as fire days during the warm season (May to September) for the period from 1979 to 2019 over California, especially for the inland region; trend analysis for the two components of VPD (saturated and actual vapor pressure) suggests that warming contributes at least 70% to the positive trend of VPD. Clustering analysis of circulation patterns with self-organizing map (SOM) shows that some patterns related to intensification of blocking ridge and high VPD condition in California are becoming more and more frequent during the period, indicating the role of circulation changes. However, for the same types of circulation pattern, VPD also increases significantly during the recent two decades (2000-2019) compared to the previous two decades (1980-1999). Using flow analogue analysis, we show that the decadal circulation variation can only explain less than half of the VPD trend in California, though it explains much of the interannual variability of VPD; these results imply the anthropogenic warming is contributing to the increase of VPD and fire activities in recent decades.